This application claims the benefit of German patent application 10009131.8 filed Feb. 26, 2000, herein incorporated by reference
The present invention relates generally to a method and a device for the optical detection of impurities, in particular foreign fibers, in a longitudinally traveling yarn, and relates more particularly to such a method wherein light is emitted in the direction of the yarn, the intensity of the transmitted light is measured and converted into a first electrical signal whose value is a function of the instantaneous diameter of the yarn, the light reflected by the yarn is also measured, and a second electrical signal is produced.
The detection of impurities is of great importance in the production of yarns. Impurities, particularly foreign fibers, can have disadvantageous effects on the final product.
It is known from European Patent Publications EP 0 197 763 and EP 0 553 545 to monitor a yarn in front of a background with the same reflecting ability as the yarn. Thus, it is intended that the amount of reflected light be essentially independent of the diameter of the yarn and a change in the reflected light does not indicate a change of the yarn diameter, but an impurity, for example as a result of foreign fibers. In this case, it is necessary to match the reflections from the background and the yarn very exactly to each other, because otherwise thick or thin places in the yarn cause changes in the reflected light and distort the results of the foreign fiber detection. The matching is elaborate, can only be automated to a limited extent or not at all, and moreover must be performed with every change in the fiber material, such as after a batch change, for example.
European Patent Publications EP 0 553 446 and EP 0 572 592 disclose methods and devices for detecting foreign fibers, wherein light from an illumination device is cast on the moving yarn, where it is reflected and transmitted. The reflected light and the transmitted light are each converted into electrical signals. These two electrical signals are linked to each other and a further electrical signal is thereby obtained, in which the foreign fibers are indicated and the other yarn defects, such as thick or thin places, are suppressed. With such signal linkages, the signals are customarily processed in the form of digital signals. Errors may occur in the value of a digital signal if rounding of the signal value is necessary in the course of converting analog signals into discrete digital signals which, in connection with the detection offoreign fibers, can lead to distortions of the measured results. In case of changes of the reflection signal, the signal portions which are dependent on the diameter can be ten, or even a hundred times, greater than the foreign fiber-dependent signal portions. Small errors during the linkage of the signals and the elimination of the diameter-dependent signal portions can lead to a considerable distortion of the smaller amount of the foreign fiber signal, and therefore of the results of the foreign fiber detection.
Another device is known from European Patent Publication EP 0 572 592, by means of which changes of the light from the light source, which are caused by dirt, are compensated by keeping the intensity of the light source constant. In this manner, it is intended to avoid changes in the light which develop over a long term or over a long time period, such as can be caused by dirt.
It is accordingly an object of the present invention to reduce the disadvantages in connection with the known methods and devices for detection of foreign fibers, and to improve the detection of impurities in longitudinally traveling yarn.
The method of the present invention seeks to achieve this object by optically detecting foreign fibers and other impurities in a longitudinally traveling yarn, through the steps of emitting a light in the direction of the yarn; measuring the intensity of the emitted light; producing a first electrical signal based on the measuring of the emitted light intensity, the first signal being of a value which is a function of the instantaneous diameter of the yarn; following the formation of the first electrical signal, adjusting the intensity of the emitted light as a function of the yarn diameter based on the first electrical signal to compensate for the effect of the yarn diameter on the light reflected by the yarn; then measuring light reflected by the yarn; producing a second electrical signal based on the measuring of the reflected light; and directly using the second electrical signal for detecting impurities in the yarn. A matching of the reflecting capability of the measuring background with the reflecting capability of the yarn is no longer required for this method. Since in accordance with the invention signal portions which depend on the diameter are not even entered into the second signal, and changes in the second signal directly indicate impurities, it is possible to omit computing steps for linking the second signal with another signal, or with a signal which to a large degree is dependent on the diameter, thus eliminating the dependency on the diameter for detecting the foreign fiber proportion. Any rounding or computational errors caused by such computing steps, as well as the summation of such errors, is prevented.
The present invention further prevents rounding errors which occur in converting analog to digital signals in that the first electrical signal is generated in the form of an analog signal. A possible, but undesired, influencing of the measured values is prevented in a simple way by registering the first and second electrical signals at a predetermined clock frequency, or by pulsing the light source at a predetermined clock frequency.
Through the present manner of controlling the intensity of the emitted light in obtaining the second electrical signal, a completely different operating mechanism exists then in comparison to the device for regulating the output of the light source known from European Patent Publication EP 0 572 592. While the known device is intended to compensate for changes and to always maintain the light intensity constant, the control of the intensity of the emitted light in the present invention for forming the second electrical signal is purposely not kept constant, in contrast to the known device, but is set as a function of the first electrical signal, and therefore as a function of the yarn diameter. Thus, in accordance with the present invention, the light intensity of the light source for forming the second electrical signal is changed when a change of the first electrical signal occurs whereby, with an increased yarn diameter, the light intensity is set correspondingly lower, and with a decreased yarn diameter, the light intensity is set correspondingly higher.
The outlay for setting the intensity of the light source as a function of the first electrical signal can be reduced by setting a constant value of the light intensity of the light source in connection with the first measurement for forming the first signal. As a result, the diameter-dependent first measurement for forming the first signal always takes place under the same conditions, and the first signals can be directly used as a measurement of the instantaneous diameter of the yarn. The first electrical signal can be employed not only for controlling the intensity of a light source, but can additionally be evaluated for monitoring thick and thin places in the yarn.
In a preferred embodiment of the method, the second electrical signal is obtained at a time delay in respect to the first electrical signal at a measurement point which is located downstream in the direction of the yarn travel, with the time delay being controlled as a function of the yarn velocity such that both signals are obtained at the same location or section along the yarn. An extremely great measurement accuracy and measurement dependability is thereby assured.
The present invention also contemplates a novel device for performing the afore-described method and, particularly, is advantageous in compensating for changes in light intensity caused by aging or soiling of the light sources.
The second electrical signal obtained in accordance with the invention leads to increased measuring sensitivity in the course of the optical detection of impurities. Critical areas, such as the differentiation between dark brown and black, or the differentiation between off-white and pure white, can be easily controlled without problems through the present invention. The invention permits a clear improvement in the detection of impurities, in particular foreign fibers, in linearly moved yarn.
Further advantageous embodiments of the invention will be understood from the following disclosure of exemplary embodiments of the invention represented in the drawings.